Add method toArray to Heap

[masx200]

fixes #3076

Returns the underlying cloned array in arbitrary order without sorting

[masx200]

I also feel that it's confusing that [...tree] and tree.toArray() have significantly different meanings. Could this be named in better way?

Please give a better name to this method. @kt3k

[kt3k]

Java, Rust, and Dart seem giving just generic names.
Java - toArray https://docs.oracle.com/javase/jp/8/docs/api/java/util/PriorityQueue.html
Rust - into_vec https://doc.rust-lang.org/std/collections/struct.BinaryHeap.html#method.into_vec
Dart - toArray https://pub.dev/documentation/pathfinding/latest/pathfinding.core.heap/Heap/toArray.html

Flutter's collection package gives explicit name.
Flutter - https://api.flutter.dev/flutter/package-collection_collection/HeapPriorityQueue-class.html

I now feel toArray might be just fine.

Does anyone have any opinion on this?

cc @KyleJune

[kt3k]

LGTM